Conventional electrical power systems utilize a synchronous electrical generator for generating AC power. Particularly, such a generator may include a rotor and a stator having a stator coil. In applications such as an aircraft, the rotor is driven by an engine so that electrical power is developed in the stator coil. Owing to the variation in engine speed, the frequency of the power developed in the generator windings is similarly variable. This variable frequency power is converted to constant frequency power in a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200 Vac power at 400 Hz. Such known converters are controlled by a generator/converter control unit (GCCU).
In order to provide aircraft engine starting, such known power systems have operated the generator as a motor. Specifically, an external power source is coupled through a start control to the generator to energize the stator coil and thus develop motive power to start the engine. The components required in such a start control increase the weight of the aircraft and take up valuable space. To minimize the size and weight of such start controls, certain known aircraft VSCF power systems have utilized the existing converter and GCCU for the start control.
In the start mode of operation, the converter may be supplied power from any 400 Hz power source, such as, for example, an auxiliary power unit generator or an external power source. However, each such power source might have a different available capacity for use in engine starting. Therefore, the GCCU must be configured to provide engine starting from any such available power sources and to limit the amount of power drawn.
Rozman et al. co-pending application entitled VSCF Start System with Selectable Input Power Limiting, U.S. patent application Ser. No. 270,625, filed Nov. 14, 1988, now U.S. Pat. No. 4,949,021, and owned by the assignee of the present invention, which is hereby incorporated by reference herein, discloses a start control which provides input power limitations in accordance with input power requirements. Specifically, the start control described therein utilizes a pulse width modulated inverter to control torque and power as functions of the output voltage and commutation angle. Specifically, the start control maintains the volts/hertz ratio at a constant and uses closed loop control of the commutation angle at speeds above a preselected minimum to control current and to limit input power.
Such a start control system utilizes open loop voltage control which assumes that the constant volt/hertz ratio is maintained. In fact, voltage may increase or decrease if the power source is not accurate.
When driving a synchronous motor at various frequencies, it is important to maintain a constant volt/hertz ratio. If this ratio is too high, then the motor may saturate. If the ratio is too low, then the motor develops less torque and less power.
Conventional voltage control schemes implement closed loop control by detecting inverter output voltage and correcting the PWM signal by the difference between this voltage and a voltage reference. However, higher harmonic components in the inverter output limit the accuracy of such schemes. Particularly, the armature component of the magnetic flux is generated by the fundamental component of the output voltage and the higher harmonics detrimentally affect the desired control scheme.
The present invention is intended to overcome one or more of the problems as set forth above.